1. Field of the Invention
The present invention relates to a field effect transistor formed of a compound semiconductor, which is satisfactorily operable in a millimeter wave band at which a field effect transistor formed of Si (silicon) is no longer operable. More specifically, the present invention relates to a 2DEG.cndot.FET (two-dimensional electron gas field effect transistor) having a channel formed of a two-dimensional electron gas created at a heterojunction.
2. Description of Related art
Recently, attention has been focused on ternary or quaternary mixed crystalline compound semiconductors such as InGaAs and InGaAsP. Among them, InGaAs which is in lattice matching with an InP substrate is promising not only as a material for photo devices but also as a material for various types of field effect transistors (FET). In particular, research for FETs utilizing a two-dimensional electron gas created at a heterojunction between an InP layer and an InAlAs layer has become extensive.
The reason for which InGaAs is considered to be more promising than GaAs as an electron transportation device material is that: (1) a peak value of an electron drift mobility is large; (2) an electron mobility in a low electric field is large.; (3) it is easy to form an ohmic contact and therefore a contact resistance is low (a barrier between InGaAs and a metal is low): (4) a larger overshoot of the electron mobility is expected under a high electric field; and (5) a diffusion noise caused by electron transition between a .GAMMA. alley and a L valley is small.
At present, a FET utilizing a two-dimensional electron gas created at an InGaAs/InAlAs boundary is considered to be promising as a high performance millimeter wave device, and is now being actively studied in various organizations. It has been already confirmed at an experience level that it is effective as a low-noise device.
For example, K. H. Dub et al reported that at a room temperature, the noise figure at 94 GHz is 1.2 dB, and the associated gain 7.2 dB ("IEEE MICROWAVE AND GUIDED WAVE LETTERS", Vol. 1, No. 5, pp 114-116, May, 1991 ). This reported device was formed on an experimental basis by using a compound in lattice matching with an InP substrate, namely, by adjusting the composition ratio of In to In.sub.0.53 Ga.sub.0.47 As/In.sub.0.52 Al.sub.0.48 As. This heterojunction creates a two-dimensional electron gas in the In.sub.0.53 Ga.sub.0.47 As layer.
In order to further elevate the characteristics, for example, G. I Ng et al reported that the characteristics of the FET is improved by making the "In" composition ratio in the InGaAs layer larger than 0.53 ("IEEE ELECTRON DEVICE LETTERS", Vol. 10, No. 3, pp 114-116, June, 1989). However, if InGaAs having the "In" composition ratio of 0.53 or more is formed on the InP substrate, a lattice mismatching occurs. Therefore, the thickness allowing a single crystal growth is restricted by the "In" composition ratio, and accordingly, the thickness of the InGaAs channel layer is limited.
In addition, T. Akazaki et al reported the FET which creates a two-dimensional electron gas having a strong confinement effect, by inserting a thin InAs layer into the InGaAs channel ("IEEE ELECTRON DEVICE LETTERS", Vol. 13, No. 6, pp 325-327, June, 1992).
From a different viewpoint, it is difficult to make an In-based material have a high resistance, and since a Schottky barrier height .phi.B at a Schottky junction is low, a property withstanding an applied voltage is lower than a GaAs-based material, and therefore, an operating voltage of the FET cannot be increased.
In order to increase the breakdown voltage, it may be considered to use a large energy gap material in the channel layer. The energy gap Eg of In.sub.0.53 Ga.sub.0.47 As in lattice matching with InP is 0.74 eV. The energy gap Eg of InGaAs with InP can be changed from 0.74 eV to 1.35 eV, by changing its composition ratio but lattice matching cannot be maintained.
W. P. Hong et al reported an experimentally manufactured HEMT having a channel layer formed of InGaAsP and an electron supply layer formed of InAlAs ("IEEE ELECTRON DEVICE LETTERS", Vol. 12, No. 10, pp 559-561, October, 1991). In the reported device, the energy gap of In.sub.0.73 Ga.sub.0.27 As.sub.0.6 P.sub.0.4 is 0.95 eV, which is larger than In.sub.0.53 Ga.sub.0.47 As in lattice matching with InP, about 0.2 eV. Accordingly, it was reported that deterioration of the breakdown voltage caused by impact ionization it suppressed in a high electric field region, and a drain-source breakdown voltage of not less than 5 V and a gate-drain breakdown voltage of not less than 15 V were obtained.
W. P. Hong et al used InGaAsP as the electron passing channel layer, but in JP-B-02-60223 Sasaki et al proposed a FET having an electron supply layer formed of InGaAsP. In place of InAlAs used in the prior art, InGaAsP is used to form the electron supply layer, with the result that the conduction band discontinuity amount .DELTA.Ec between the InGaAsP electron supply layer and the InGaAs channel layer can be made small. Thus, an enhancement type FET was manufactured on an experimental basis.
As mentioned above, it is disadvantageous in that the breakdown voltage of the InAlAs/InGaAs heterojunction FET is low, and therefore, a large operating bias voltage cannot be used. On the other hand, since the InGaAsP channel layer has an energy gap larger than the conventional InGaAs channel layer, the deterioration of the FET breakdown voltage caused by the impact ionization is suppressed. However, the conduction band discontinuity amount .DELTA.Ec between the InAlAs electron supply layer and the InGaAsP channel layer can be made smaller than that between the InAlAs electron supply layer and the InGaAs channel layer, and therefore, a sheet electron density of the two-dimensional electron gas, where a quantum well is formed, becomes small. Therefore, there arises another problem that a sufficient carrier concentration cannot be obtained. Furthermore, since the conduction band discontinuity amount .DELTA.Ec is small, the effect of confining the two-dimensional electron gas is weaker than that of the prior art. As a result, the carriers leak to the InAlAs electron supply layer, and therefore, transconductance of the FET is deteriorated.